Data transmission device and method

ABSTRACT

A data transmission device for generating a plurality of compressed/encoded data of different bit rates from a single video signal, whereby unevenness in the amount of generated data can be easily reduced. A synchronizing signal detection section detects a synchronizing signal from the input video signal and supplies the detected signal to a timing control section. Compressing/encoding sections compress/encode the same video signal input thereto to generate data streams of different bit rates. The timing control section controls the compressing/encoding sections in accordance with the synchronizing signal detected by the synchronizing signal detection section such that timings for starting compression/encoding processes in the compressing/encoding sections are offset in units of frame. A multiplexing section generates fragmented packets carrying the individual data streams in accordance of amounts of data generated per unit time by the respective compressing/encoding sections, and sequentially transmits the fragmented packets at equal intervals within the unit time.

BACKGROUND OF THE INVENTION

[0001] (1) Field of the Invention

[0002] The present invention relates to a data transmission device andmethod for generating a plurality of compressed/encoded data ofdifferent bit rates from a single video signal and simultaneouslytransmitting the generated data onto a network, and more particularly,to a data transmission device and method applicable to real-timetransmission of such compressed/encoded data.

[0003] (2) Description of the Related Art

[0004] Recent image compression/encoding techniques such as MPEG (MovingPicture Experts Group) have made it easy to deliver moving picture dataover networks. However, in the case of delivering such data through theInternet in particular, the delivered data may possibly be transferredvia analog telephone lines or ISDN (Integrated Services Digital Network)lines, and broadband communication is not necessarily available to everyrecipient. Under the present circumstances, therefore, it is necessarythat the resolution be lowered or the compression ratio be increased topermit data to be delivered at a relatively low bit rate.

[0005] In view of this, a moving picture data delivery scheme has beenconceived wherein two types of data, that is, one for delivery to arelatively broadband network, such as an intranet in a corporation, andthe other for delivery to a relatively narrowband network, such as theInternet, are generated from a single video source and are deliveredsimultaneously. For example, for a broadband network, a data streamcompressed/encoded according to MPEG-2 is delivered at a bit rate ofabout 6 Mbps, and for a narrowband network, a data streamcompressed/encoded according to MPEG-4 is delivered at a bit rate ofabout 100 kbps.

[0006] Heretofore, when generating a plurality of data streams fordelivery at different bit rates from a single video source, encodersequal in number to the data streams to be generated are used to encodedata distributed from the video source. Alternatively, a transcoder orthe like is used to decode the data stream for broadband delivery andthen to again encode the decoded data stream to obtain a data stream fornarrowband delivery.

[0007] However, the delivery of moving picture data has now become sopopularized that there is a strong demand for reduction in cost ofdeliverer-side systems as well as in size of such systems to saveinstallation space. Also, in recent years, real-timeliness orsimultaneity of delivered data is often given importance especially incases where the water levels of rivers or dams, roads, etc. aremonitored from a remote location or a conference or a concert isbroadcast live. Accordingly, there has been proposed an idea ofincorporating a plurality of encoder engines into a single encoder, togenerate a plurality of data streams of different bit rates and deliverthe generated data streams simultaneously.

[0008] Meanwhile, in the case of data which has been compressed/encodedby using inter-frame prediction as in MPEG, an appreciable differenceoften occurs between the data amount of a picture which can be decodedby its own data only and the data amount of a picture which has beengenerated using the inter-frame prediction. Accordingly, the processingload greatly varies during the image encoding/decoding process, and alsowhen such data is transmitted over a network, an actual amount oftransmitted data can momentarily rise well above the average bit rate.

[0009] As regards techniques for generating data by encoding individualobjects of image and then multiplexing the encoded objects, there hasbeen proposed a method in which the start timings for encoding objectsare offset in accordance with the ranges of variations in the amount ofcode generated per frame for the individual objects, to smooth variationin the amount of generated code as well as in the processing load (e.g.,Japanese Unexamined Patent Application No. H10-023427 (cf. ParagraphNos. [0037] to [0051], FIG. 5)).

[0010] Thus, the compression/encoding techniques using the inter-frameprediction as in MPEG are associated with a problem that the amount ofgenerated code varies over a wide range, as mentioned above. Especiallyin the case where a plurality of compressed/encoded data of differentbit rates are generated from a single video source and are deliveredsimultaneously, the amount of generated code varies over an even widerrange, giving rise to a problem that data cannot be received properlywhere the amount of data transmitted onto the network momentarilyincreased.

[0011]FIG. 9 illustrates variation in the amount of data observed when aplurality of compressed/encoded data of different bit rates aresimultaneously delivered, wherein FIG. 9(A) shows exemplary arrangementsof pictures in respective data streams generated according to MPEG-2,and FIG. 9(B) is a graph showing a total amount of data generated withrespect to each picture.

[0012] In FIG. 9(A) is illustrated the case where two data streams A andB of different bit rates are generated from a single video source by twoencoders. A data stream encoded according to MPEG-2 (or MPEG-1)comprises an I picture encoded in a closed manner within a frame, a Ppicture encoded using forward prediction, and a B picture encoded usingbidirectional prediction. The data streams A and B shown in FIG. 9(A)have a general picture arrangement in which one I or P picture ispreceded and followed by two B pictures. A GOP (Group Of Pictures) is aunit that allows playback of the data stream in the middle, and one GOPalways includes one or more I pictures. In the illustrated example, afixed number of pictures constitutes one GOP.

[0013] Since the I picture is generated by closed encoding within aframe, its data amount is noticeably large, compared especially with theB picture. In a data stream having a picture arrangement as shown inFIG. 9(A), the data amount of I pictures accounts for nearly ⅓ of thetotal amount of the data stream.

[0014] In the illustrated example, every twelve pictures include one Ipicture, and therefore, in terms of an average data delivery rate persecond, half of the amount of data delivered during a period of{fraction (11/12)} second is delivered within the remaining period of{fraction (1/12)} second at a time. Where the data stream has an averagebit rate of 6 Mbps, for example, there is a possibility that data isgenerated at an instantaneous rate of 24 Mbps when an I picture isgenerated. Further, since the bit rate of 24 Mbps is a value thatapplies to the case where data is delivered uniformly during a period of{fraction (1/12)} second, data can possibly be transmitted at an evenhigher rate if the data is transmitted at a time as soon as it isgenerated.

[0015] Also, in the case where multiple data streams are encodedsimultaneously by multiple encoder engines incorporated in a singleencoder, the encoding processes are usually started at the same time andI, B and P pictures are generated at respective identical positions, asshown in FIG. 9(A). Consequently, at the timing when I pictures aregenerated for the respective data streams, the total amount of generateddata sharply increases for a moment. For example, where the average bitrates of the data streams A and B are 6 Mbps and 3 Mbps, respectively,the data generation rate reaches 36 Mbps (=24 Mbps+12 Mbps) at its peak.Such extreme unevenness in the amount of generated data instantaneouslyincreases the load on the network for transmitting data and causespacket loss etc.

[0016] It is possible to generate encoded data in such a manner as toreduce unevenness in the amount of generated data. In this case,however, it is necessary that the encoder be provided therein with alarge-capacity buffer to encode data while temporarily storing aconsiderable amount of data. As a result, the transmission of data isdelayed behind the original video, thus impairing the simultaneity.Also, complex control is required, which leads to an increase in cost ofthe device.

SUMMARY OF THE INVENTION

[0017] The present invention was created in view of the abovecircumstances, and an object thereof is to provide a data transmissiondevice which permits unevenness in the amount of generated data to bereduced with ease in the case where a plurality of compressed/encodeddata of different bit rates are generated from a single video signal.

[0018] Another object of the present invention is to provide a datatransmission method which permits unevenness in the amount of generateddata to be reduced with ease in the case where a plurality ofcompressed/encoded data of different bit rates are generated from asingle video signal.

[0019] To achieve the first object, there is provided a datatransmission device for generating a plurality of compressed/encodeddata of different bit rates from a single video signal andsimultaneously transmitting the compressed/encoded data onto a network.The data transmission device comprises a synchronizing signal detectionsection for detecting a synchronizing signal from the video signal inputthereto, a plurality of compressing/encoding sections forcompressing/encoding the video signal to generate data streams ofdifferent bit rates, respectively, a timing control section forcontrolling the compressing/encoding sections in accordance with thedetected synchronizing signal such that timings for startingcompression/encoding processes in the compressing/encoding sections areshifted from one another in units of frame, and a multiplexing sectionfor sequentially multiplexing the data streams generated by therespective compressing/ encoding sections and transmitting themultiplexed data onto the network.

[0020] Also, to achieve the second object, there is provided a datatransmission method for generating a plurality of data streams ofdifferent bit rates by compressing/encoding a single video signal andfor simultaneously transmitting the data streams onto a network. Thedata transmission method comprises the step of detecting a synchronizingsignal from the input video signal, the step of shifting start timingsfor compression/encoding processes corresponding to the generation ofthe respective data streams from one another in units of frame inaccordance with the detected synchronizing signal, and the step ofgenerating fragmented packets carrying the individual data streams inaccordance with amounts of data generated per unit time by therespective compression/encoding processes, and transmitting thefragmented packets onto the network at equal intervals within the unittime.

[0021] The above and other objects, features and advantages of thepresent invention will become apparent from the following descriptionwhen taken in conjunction with the accompanying drawings whichillustrate preferred embodiments of the present invention by way ofexample.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIGS. 1(A) to 1(C) illustrate the principle of the presentinvention;

[0023]FIG. 2 is a diagram showing a system configuration of an imagedelivery system to which a data transmission device of the presentinvention is applicable;

[0024]FIG. 3 is a block diagram illustrating functions of a datatransmission device according to one embodiment of the presentinvention;

[0025]FIG. 4 is a diagram showing a configuration of a signal system forcontrolling operation start timings for individual encoders;

[0026] FIGS. 5(A) to 5(E) are a time chart showing waveforms of signalssent to the individual encoders from a video timing section;

[0027] FIGS. 6(A) and 6(B) illustrate arrangements of pictures inrespective generated data streams and amounts of generated data;

[0028] FIGS. 7(A) and 7(B) illustrate data amounts of respectivepictures in a generated data stream and packets carrying the pictures;

[0029]FIG. 8 is a flowchart illustrating a process performed on each ofthe data streams by a packet generating section; and

[0030] FIGS. 9(A) and 9(B) illustrate variation in data amount observedwhen a plurality of compressed/encoded data of different bit rates aredelivered simultaneously.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Embodiments of the present invention will be hereinafterdescribed with reference to the drawings.

[0032]FIG. 1 illustrates the principle of the present invention.

[0033] A data transmission device 1 of the present invention is a devicefor compressing/encoding a video signal from a camera or the like anddelivering a generated data stream in real time onto a network 2. Inthis case, since networks configured up to recipients can be eitherbroadband or narrowband, a plurality of data streams of different bitrates are generated from a single video source, so as to match with thedifferent bands, and are delivered simultaneously.

[0034] As shown in FIG. 1(A), the data transmission device 1 comprises asynchronizing signal detection section 11 for detecting a synchronizingsignal from a video signal input thereto, compressing/encoding sections12 a and 12 b for compressing/encoding the input video signal togenerate data streams of different bit rates, respectively, a timingcontrol section 13 for controlling the compressing/encoding sections 12a and 12 b such that timings for starting compression/encoding processesin the compressing/encoding sections 12 a and 12 b are shifted from eachother in units of frame, and a multiplexing section 14 for sequentiallymultiplexing the data streams generated by the respectivecompressing/encoding sections 12 a and 12 b and transmitting themultiplexed data onto the network 2. The example shown in FIG. 1includes only two compressing/encoding sections, but three or morecompressing/encoding sections may be provided.

[0035] The synchronizing signal detection section 11 detects asynchronizing signal necessary for the detection of start timing for aframe or field or horizontal scanning, demodulation of chrominancesignals, etc., from the input video signal. In the case where NTSC(National TV Standards Committee) composite signal is input as the videosignal, for example, vertical synchronizing signal, horizontalsynchronizing signal, color synchronizing (color burst) signal, etc. aredetected as the synchronizing signal. The detected synchronizing signalis supplied to the timing control section 13.

[0036] Using the thus-supplied synchronizing signal, the timing controlsection 13 controls the timings for starting the compression/encodingprocesses in the respective compressing/encoding sections 12 a and 12 b.In this case, the timings for starting the compression/encodingprocesses in the compressing/encoding sections 12 a and 12 b arecontrolled so as to be shifted from each other in units of frame.

[0037] Under the control of the timing control section 13, thecompressing/encoding sections 12 a and 12 b compress/encode the inputvideo signal at respective different bit rates, to generate movingpicture data streams.

[0038] The multiplexing section 14 sequentially packetizes andmultiplexes the data streams generated by the compressing/encodingsections 12 a and 12 b, and transmits the multiplexed data onto thenetwork 2. Also, the multiplexing section 14 carries out control suchthat fragmented packets are generated in accordance with the amount ofdata generated by the individual compressing/encoding sections 12 a and12 b and are transmitted at equal intervals within a unit time.

[0039] The compressing/encoding sections 12 a and 12 b are provided inthe data transmission device 1 correspond in number to data streams tobe generated and are operated in parallel. Thus, although there is a“shift” between the start timings for generating the respective datastreams, no delay occurs in the subsequently generated data streams,making it possible to deliver image with remarkably enhancedsimultaneity.

[0040] Operation of the data transmission device 1 will be now describedon the assumption that NTSC (National TV Standards Committee) compositesignal is input as the video signal.

[0041] The video signal input to the data transmission device 1 issupplied to the two compressing/encoding sections 12 a and 12 b as wellas to the synchronizing signal detection section 11. Each of thecompressing/encoding sections 12 a and 12 b receives, as the videosignal, a digital video signal which has been obtained by processing thevideo signal in an A/D conversion section etc., not shown, and starts tocompress/encode the video signal in accordance with a control signalfrom the timing control section 13.

[0042] The compressing/encoding sections 12 a and 12 b generate datawith different compression ratios and different resolutions, butpictures are generated at the same intervals. In the case where thecompression/encoding is performed using inter-frame prediction, thearrangements of pictures in the data streams generated by the respectivecompressing/encoding sections 12 a and 12 b are usually the same.

[0043] Thus, by shifting the start timings for the compression/encodingprocesses from each other in units of frame, the picture arrangements ofthe data streams generated by the respective compressing/encodingsections 12 a and 12 b can be made to differ from each other. As aconsequence, the data amount of generated picture sharply increases atdifferent timings between the data streams, whereby variation in theoverall data amount of the generated data streams can be smoothed.

[0044] On the other hand, the synchronizing signal detection section 11detects, as the synchronizing signal, a vertical synchronizing signal, acolor synchronizing signal, etc. from the video signal, and outputs thedetected signals to the timing control section 13. The timing controlsection 13 acquires frame (or field) start timing from the verticalsynchronizing signal input thereto, and also generates a chrominancesubcarrier signal synchronized with the color synchronizing signal. Thechrominance subcarrier signal is used as a reference signal whenseparating chrominance signals in the input video signal, etc.

[0045] In the case of NTSC signal, the phase of the chrominancesubcarrier signal is inverted every two fields (i.e., every frame).Accordingly, the timing control section 13 can easily acquire the shiftamount corresponding to one frame by detecting the phase of thechrominance subcarrier signal at the frame start timing, as shown inFIG. 1(B), and can provide the compressing/encoding sections 12 a and 12b with the start timings for their respective compression/encodingprocesses. Specifically, when the frame start timing synchronizes withthe rise timing of the chrominance subcarrier signal, thecompressing/encoding section 12 a is instructed to start the process,and when the frame start timing thereafter synchronizes with the falltiming of the chrominance subcarrier signal, the compressing/encodingsection 12 b is instructed to start the process. This serves to smoothvariation in the total data amount of the generated data streams.

[0046] The generated data streams are output to the multiplexing section14. As shown in FIG. 1(C), when the amount of data generated per unittime by either of the compressing/encoding sections 12 a and 12 b is toolarge, the multiplexing section 14 generates fragmented packets carryingsuch data. For example, a reference value is set for the data amount tobe carried by one packet, and if the amount of data corresponding to onepicture (i.e., one frame) in either of the data streams exceeds thereference value, the picture data is fragmented into a plurality ofpackets. The thus-fragmented packets are transmitted at equal intervalswithin a unit time.

[0047] The packet transmission control described above makes it possibleto further smooth variation in the amount of data transmitted to thenetwork 2, to lower the peak bit rate of transmitting data, and toreduce the transmission load. The reference value specifying the dataamount to be carried by one packet may be set as desired, taking accountof the performance of the data transmission device 1 itself and thecapacity of the network 2.

[0048] As described above, according to the present invention, thesynchronizing signal detected from the input video signal is used sothat the timings for starting the compression/encoding processes in therespective compressing/encoding sections 12 a and 12 b may be shiftedfrom each other in units of frame. Consequently, a sudden increase inthe amount of generated data takes place at scattered timings in therespective data streams, and since variation in the total amount oftransmitting data can be smoothed as a result, packet loss etc. can beprevented from occurring due to increase in the network load.

[0049] Especially in the case where NTSC composite signal is used as theinput, control timings for the compressing/encoding sections 12 a and 12b can be derived with ease from the vertical synchronizing signal andcolor synchronizing signal detected from the input video signal. It istherefore possible to control the start timings for thecompression/encoding processes without the need to use a counter circuitetc. for obtaining a shift amount of frame start timings, for example,and accordingly, the device can be reduced in cost and size.

[0050] Further, in accordance with the data amount of the data streamgenerated by each of the compressing/encoding sections 12 a and 12 b,the multiplexing section 14 generates fragmented packets carrying thedata stream, and transmits the fragmented packets at equal intervals,whereby variation in the amount of data transmitted onto the network 2can be made even smoother. In this manner, the present invention usesthe start timing control for the compression/encoding processes incombination with the packetization control for generated data, wherebythe effect of smoothing variation in the amount of data transmitted ontothe network 2 can be enhanced without impairing simultaneity oftransmitted data.

[0051] In the example described above, the timings for starting thecompression/encoding processes are shifted from each other by one frame,but the start timings may be shifted from each other by two or moreframes. Also, where three or more data streams of different bit ratesare generated simultaneously, the process start timings of therespective compressing/encoding sections may be sequentially shifted inunits of frame. In this case, however, the shift amount is preferablyset to one frame, in order to more securely smooth variation in theamount of generated data.

[0052] An embodiment of the present invention will be now described indetail. In the following description, the invention is applied, by wayof example, to a system for delivering image to recipients connected toan intranet such as a LAN (Local Area Network) in a corporation, or tothe Internet.

[0053]FIG. 2 shows a system configuration of an image delivery system towhich the data transmission device of the present invention can beapplied.

[0054] The image delivery system shown in FIG. 2 generates data streamsfrom images acquired by cameras 110 a and 120 a and delivers thegenerated data streams in real time to recipients through networks. Theimage delivery system is used, for example, to monitor the water levelsof rivers or dams, roads, etc. from a remote location or to broadcast aconference or a concert live.

[0055] The image delivery system comprises data transmission devices 110and 120 to which the cameras 110 a and 120 a are connected,respectively, and receiving terminals 210 and 220 for receiving movingpicture data streams delivered from the data transmission devices 110and 120. The data transmission devices 110 and 120 are connected throughan intranet 310 to the Internet 320. The receiving terminal 210 isconnected to the intranet 310, while the receiving terminal 220 isconnected to the Internet 320 through, for example, a telephone line,not shown.

[0056] The cameras 110 a and 120 a each acquire an image and output theacquired image as NTSC composite signal. The data transmission devices110 and 120 compress/encode the image signals from the respectivecameras 110 a and 120 a in accordance with MPEG, and transmit thecompressed/encoded data onto the intranet 310. The receiving terminals210 and 220 have the function of receiving the thus-transmitted datastreams through the intranet 310 and the Internet 320, respectively,decoding and displaying the data, and each comprise, for example, acomputer such as a PC (Personal Computer).

[0057] The system may include larger numbers of data transmissiondevices and receiving terminals than illustrated. However, the datatransmission devices need to be connected to the intranet 310.

[0058] The intranet 310 is a network that warrants high-speed datatransmission/reception as a whole, compared with the Internet 320. Thedata transmission device 110 generates, based on a single video sourcefrom the camera 110 a, data streams of different bit rates matching therespective transmission bands of the intranet 310 and Internet 320, andtransmits the generated data streams. This is the case with the datatransmission device 120.

[0059] Accordingly, of the data streams delivered from the datatransmission device 110, for example, a data stream A with a relativelyhigh bit rate of several Mbps can be received properly by the receivingterminal 210, and a data stream B with a relatively low bit rate ofseveral hundred kbps can be received properly by the receiving terminal220, as shown in FIG. 2. Each of the data transmission devices 110 and120 transmits data by multicasting, for example, and the receivingterminals 210 and 220 can select the bit rate at which they are toreceive the data stream.

[0060] Processing functions of the data transmission device 110, as anexample, will be now described. FIG. 3 is a block diagram illustratingthe functions of the data transmission device 110.

[0061] As shown in FIG. 3, the data transmission device 110 comprises anA/D conversion section 111, a sync detection section 112, a low-passfilter 113, a Y/C separation section 114, a filtering/scaling section115, an output formatting section 116, encoders 117 a and 117 b, a videotiming section 118, and a packet generating section 119.

[0062] The A/D conversion section 111 samples the NTSC composite signaltransmitted from the camera 110 a and converts the signal to a digitalvideo signal. The sync detection section 112 detects a verticalsynchronizing signal, a horizontal synchronizing signal and a colorsynchronizing signal from the digital video signal converted by the A/Dconversion section 111, and outputs the detected signals to the videotiming section 118.

[0063] The low-pass filter 113 passes only low-frequency components ofthe video signal from the A/D conversion section 111, to thereby removenoise. The Y/C separation section 114 separates the video signalsupplied from the low-pass filter 113 into a luminance signal and colordifferential signals.

[0064] The filtering/scaling section 115 performs resolution conversionand effective image area cropping on the video signal which has beensubjected to the Y/C separation. The output formatting section 116buffers, on a frame-by-frame basis, the video signal supplied from thefiltering/scaling section 115, and provides a non-interlaced videosignal.

[0065] The encoders 117 a and 117 b each receive the video signal fromthe output formatting section 116 and, in accordance with respectivepredetermined resolutions and compression ratio settings,compress/encode the video signal according to MPEG-2. In this case, theencoder 117 a generates a data stream A with a relatively high averagebit rate of several Mbps, for example, and the encoder 117 b generates adata stream B with a relatively low average bit rate of several hundredkbps, for example. The encoders 117 a and 117 b generate pictures inresponse to the same field start signal from the video timing section118, as described later, and accordingly, no delay of data occurs as aresult of the compression/encoding processes by these encoders, thuspermitting very nearly real-time generation of data with respect to theoriginal video signal.

[0066] The video timing section 118 receives the synchronizing signalsdetected by the sync detection section 112 and controls the operationtimings of the encoders 117 a and 117 b, etc. Using the colorsynchronizing signal from the sync detection section 112, the videotiming section 118 generates a 14.318-MHz synchronizing signal, which isa reference signal for the synchronization of video signal within thedata transmission device 1. This synchronizing signal is used as asampling frequency by the A/D conversion section 111, for example, andalso the vertical and horizontal synchronizing signals from the syncdetection section 112 are synchronized with this synchronizing signal.Further, the video timing section 118 generates a chrominance subcarriersignal (3.5785 MHz) synchronized with this synchronizing signal.

[0067] The video timing section 118 generates field and frame startsignals based on the vertical synchronizing signal, as described later,and also outputs the chrominance subcarrier signal in order to controlthe frame start timings and compression/encoding start timings of theindividual encoders 117 a and 117 b by means of these signals. Adetailed configuration for controlling such operation start timings willbe described with reference to FIG. 4.

[0068] On receiving the data streams generated by the respectiveencoders 117 a and 117 b, the packet generating section 119 assemblesthe data into IP (Internet Protocol) packets, multiplexes the packets,and then sends the multiplexed packets onto the intranet 310.

[0069]FIG. 4 illustrates the configuration of a signal system forcontrolling the operation start timings of the encoders 117 a and 117 b.

[0070] To control the operation start timings of the encoders 117 a and117 b, the video timing section 118 outputs the field start signal Sfd,frame start signal Sfm and chrominance subcarrier signal Sc.

[0071] The field start signal Sfd is a pulse signal synchronized withrise timing of the vertical synchronizing signal, and provides theencoders 117 a and 117 b with field start timing.

[0072] The frame start signal Sfm is output once for every two pulses ofthe field start signal Sfd and is input to two AND gates 118 a and 118b. The other input terminal of the AND gate 118 a is input with thechrominance subcarrier signal Sc, and the other input terminal of theAND gate 118 b is input with a phase-inverted signal of the chrominancesubcarrier signal Sc. Output signals of the AND gates 118 a and 118 bare input to the encoders 117 a and 117 b, respectively, therebyproviding the encoders 117 a and 117 b with their respectivecompression/encoding start timings.

[0073]FIG. 5 is a time chart showing the waveforms of signalstransmitted to the encoders 117 a and 117 b from the video timingsection 118.

[0074] In an NTSC system, the field period and the chrominancesubcarrier signal maintain a relationship such that they aresynchronized once in every four fields, and the phase of the chrominancesubcarrier signal is inverted every two fields (i.e., every frame).Accordingly, if timing T501 at which a pulse of the frame synchronizingsignal Sfm is output as shown in FIG. 5(A) coincides with rise timing ofthe chrominance subcarrier signal Sc shown in FIG. 5(B), the timing T502at which the next pulse of the frame synchronizing signal Sfm is outputcoincides with fall timing of the chrominance subcarrier signal Sc.

[0075] Thus, when the compressing/encoding sections 12 a and 12 b areinstructed to start their processes, the start signal Sa is output fromthe AND gate 118 a at the timing T501 when the frame start signal Sfm isoutput for the first time after the reception of the instruction, asshown in FIG. 5(C). In response to the start signal Sa, the operation ofthe encoder 117 a is started.

[0076] The AND gate 118 b is input with the phase-inverted signal of thechrominance subcarrier signal Sc, and at the next frame start timingT502, rise timings of the phase-inverted signal and frame start signalSfm coincide with each other, as shown in FIG. 5(D). Thus, the startsignal Sb is output from the AND gate 118 b, as shown in FIG. 5(E), andthe operation of the encoder 117 b is started. As a result, theoperation start timing of the encoder 117 b is delayed by one frame fromthat of the encoder 117 a.

[0077] In the illustrated example, the frame start signal Sfm is usedfor comparison with the phase of the chrominance subcarrier signal Sc,but the field start signal Sfd may be used instead.

[0078]FIG. 6 illustrates picture arrangements of individual data streamsand amounts of data generated when the operation start timings arecontrolled in the manner described above.

[0079] The data streams A and B generated by the encoders 117 a and 117b, respectively, have a picture arrangement such that one I or P pictureis preceded and followed by two B pictures, as shown in FIG. 6(A). Also,in both data streams A and B, one GOP includes the same number ofpictures.

[0080] Since the timing for starting the compression/encoding process inthe encoder 117 b is delayed by one frame from the operation starttiming of the encoder 117 a, the individual pictures appearing in thegenerated data streams A and B are shifted from each other by one frame,as shown in FIG. 6(A). Thus, the I or P pictures of the data streams Aand B invariably appear at different positions.

[0081]FIG. 6(B) shows the amount of data generated for each picture inthe individual data streams A and B. As seen from the figure, in bothdata streams A and B, the data amount of I picture is especially large,compared with the data amount of B picture. However, since the Ipictures of the data streams A and B appear at different positions,variation in the total amount of data generated per frame in the twodata streams can be smoothed, compared with the case where the Ipictures are generated at the same time.

[0082] The timings for starting the respective compression/encodingprocesses are controlled by the video timing section 118 in theaforementioned manner, whereby variation in the total data amount of thedata streams generated by the encoders 117 a and 117 b can be smoothed.To this end, the video timing section 118 uses control signals which aregenerated based on the synchronizing signals included in the originalvideo signal. The control signals used in this case are thesynchronizing signals which are generated also in conventional devicesto cause the encoders 117 a and 117 b to start processing the digitalvideo signal converted from an input analog video signal. Therefore, theaforementioned timing control for the compression/encoding processes canbe implemented by simply adding circuitry including AND gates, and thuswith a simple structure, whereby increase in the cost of the device andin the installation space therefor can be minimized.

[0083] The following describes how variation in the amount oftransmitting data is smoothed by the packet generating section 119. FIG.7 illustrates the data amounts of respective pictures generated in thedata stream A or B and packets carrying the pictures.

[0084]FIG. 7(A) shows the data amount of each picture in the data streamA generated by the encoder 117 a, by way of example. As seen from thefigure, the I picture shows an extremely large data amount within thedata stream, as compared with the B or P picture. Thus, the packetgenerating section 119 sets a reference value D1 as an upper limit forthe data amount to be carried by one packet. In the figure, D2 indicatesa data amount which is twice the reference value D1.

[0085] In the illustrated example, the data amounts of I pictures aregreater than the reference value D1, and in such cases, the packetgenerating section 119 fragments the picture data into a plurality ofpackets. FIG. 7(B) shows the amounts of data carried by respectivepackets and the timings for transmitting the packets.

[0086] As shown in FIG. 7(B), a picture whose data amount does notexceed the reference value D1 is contained in one packet and transmittedat the frame period. On the other hand, for a picture whose data amountis larger than the reference value D1, an amount of picture datacorresponding to the reference value D1 as the upper limit is containedin one packet while the remaining data is contained in a separatepacket(s). In the case of the I pictures shown in the figure, their dataamount exceeds twice the reference value D1, and accordingly, each Ipicture is fragmented and contained in three packets.

[0087] The packets fragmented in this manner are sequentiallytransmitted at respective timings which are obtained by equally dividingthe period up to the next packet transmission timing for transmittingthe subsequent picture by the number of generated or fragmented packets.This permits a large amount of I picture data to be transmitted in adistributed manner within the transmission period allocated to onepicture, making it possible to prevent packet loss etc. from beingcaused due to sudden increase in the transmission load on the network(intranet 310).

[0088]FIG. 8 is a flowchart showing a process performed on each of thedata streams A and B by the packet generating section 119.

[0089] The packet generating section 119 is provided, for example, withbuffers for receiving input data from the encoders 117 a and 117 b,respectively. In Step S801, data corresponding to one frame, that is,one picture, is read from the buffer, and in Step S802, the data amountof the picture thus read out is detected.

[0090] Then, in Step S803, the detected data amount is compared with thepacketization reference value D1, to calculate the number of packets tobe generated. If the detected data amount exceeds n times the referencevalue D1 and at the same time is smaller than or equal to (n+1) timesthe reference value, (n+1) is set as the number of packets to begenerated to carry the data.

[0091] In Step S804, an interval for transmitting the fragmented packetsis calculated from the calculated number of packets. In the case where(n+1) has been set as the number of packets as mentioned above, a valueobtained by dividing {fraction (1/30)} second, which is the transmissioninterval allocated to one picture, by the number of packets, (n+1), isset as the interval for transmitting the fragmented packets. The packetgenerating section 119 is also provided with a timer for counting thepacket transmission interval, and sets the timer to count the calculatedtransmission interval.

[0092] Subsequently, in Step S805, predetermined header information andthe like are affixed to the data corresponding in amount to thereference value D1 to generate a first UDP (User Datagram Protocol)packet, and transmits the packet at predetermined frame synchronizationtiming onto the intranet 310.

[0093] In Step S806, it is determined whether or not all of the packetscorresponding to one picture have been transmitted. If all of thepackets have been transmitted, the process for this picture is ended,whereupon the process is again executed from Step S801 to transmit thenext picture. On the other hand, if there is a packet or packets whichare not transmitted yet, the process proceeds to Step S807.

[0094] In Step S807, the timer count is monitored to wait until a timeperiod corresponding to the transmission interval set in Step S804elapses. Upon lapse of the set time period, the process returns to StepS805 to generate and transmit the next packet. Steps S805 to S807 arerepeated thereafter until transmission of all packets corresponding toone picture is completed.

[0095] The process shown in FIG. 8 is a process performed on one datastream. With respect to the packets corresponding to the two pictures ofthe respective data streams A and B, the packet generating section 119continuously transmits the packets at the timing of the same frameperiod. Because of the process start timing control carried out by thevideo timing section 118, an I picture, which has a large data amount,is never generated simultaneously in both data streams A and B. Usually,therefore, packet fragmentation is not simultaneously carried out inboth data streams A and B. Thus, when picture data of the data stream A,for example, is fragmented, the packet of the data stream B may becontinuously transmitted immediately after the first packet of the datastream A is transmitted.

[0096] The reference value D1 specifying the data amount to be carriedby one packet can be changed as desired. Accordingly, the datatransmission amount can be controlled appropriately taking account ofthe performance of the data transmission device 110 itself as well asthe capacity and communication state of the intranet 310 to which datais transmitted.

[0097] Thus, in the embodiment described above, the timings for startingthe respective compression/encoding processes are controlled by thevideo timing section 118 while at the same time the transmission iscontrolled by the packet generating section 119 such that fragmentedpackets are generated in accordance with the amount of generated dataand are transmitted at equal intervals, whereby variation in the amountof transmitting data can be easily smoothed without impairingsimultaneity of transmitting data.

[0098] As described above, in the data transmission device of thepresent invention, the timing control section controls the timings forstarting the compression/encoding processes in the respectivecompressing/encoding sections so as to be offset in units of frame.Accordingly, the compressing/encoding sections generate data streamshaving respective different picture arrangements, and thus, the dataamounts of generated pictures increase and decrease at different timingsin the respective data streams, making it possible to smooth variationin the total amount of data transmitted to the network. Also, the timingcontrol of the timing control section is carried out based onsynchronizing signals detected by the synchronizing signal detectionsection, whereby the device can be simplified in structure. Further, themultiplexing section generates fragmented packets carrying theindividual data streams in accordance with amounts of data generated perunit time by the respective compressing/encoding sections and transmitsthe fragmented packets at equal intervals within the unit time, wherebyvariation in the amount of data transmitted to the network can be madeeven smoother.

[0099] The foregoing is considered as illustrative only of theprinciples of the present invention. Further, since numerousmodifications and changes will readily occur to those skilled in theart, it is not desired to limit the invention to the exact constructionand applications shown and described, and accordingly, all suitablemodifications and equivalents may be regarded as falling within thescope of the invention in the appended claims and their equivalents.

What is claimed is:
 1. A data transmission device for generating aplurality of compressed/encoded data of different bit rates from asingle video signal and simultaneously transmitting thecompressed/encoded data onto a network, comprising: a synchronizingsignal detection section for detecting a synchronizing signal from thevideo signal input thereto; a plurality of compressing/encoding sectionsfor compressing/encoding the video signal to generate data streams ofdifferent bit rates, respectively; a timing control section forcontrolling said compressing/encoding sections in accordance with thedetected synchronizing signal such that timings for startingcompression/encoding processes in said compressing/encoding sections areshifted from one another in units of frame; and a multiplexing sectionfor sequentially multiplexing the data streams generated respectively bysaid compressing/encoding sections and transmitting the multiplexed dataonto the network.
 2. The data transmission device according to claim 1,wherein the input video signal comprises an NTSC composite signal, andsaid synchronizing signal detection section detects a verticalsynchronizing signal and a color synchronizing signal from the NTSCcomposite signal.
 3. The data transmission device according to claim 2,wherein said timing control section causes one of saidcompressing/encoding sections to start the compression/encoding processwhen frame start timing of the video signal derived based on thevertical synchronizing signal coincides with rise timing of achrominance subcarrier signal synchronized with the color synchronizingsignal, and causes a different one of said compressing/encoding sectionsto start the compression/encoding process when the frame start timingcoincides thereafter with fall timing of the chrominance subcarriersignal.
 4. The data transmission device according to claim 1, whereinsaid multiplexing section generates fragmented packets carrying theindividual data streams in accordance with amounts of data generated perunit time by said compressing/encoding sections, respectively, andtransmits the fragmented packets at equal intervals within the unittime.
 5. The data transmission device according to claim 4, wherein saidmultiplexing section sets a reference amount of data to be carried byone packet, and if an amount of data generated by any one of saidcompressing/encoding sections during a data generation periodcorresponding to one frame exceeds n times (n is an integer greater thanzero) the reference amount, said multiplexing section fragments andcarries the generated data into (n+1) fragmented packets each having adata amount equal to or smaller than the reference amount andsequentially transmits the fragmented packets at equal intervalsobtained by equally dividing the data generation period by the number ofthe fragmented packets.
 6. The data transmission device according toclaim 5, wherein said reference amount can be set to a desired value. 7.A data transmission method for generating a plurality of data streams ofdifferent bit rates by compressing/encoding a single video signal andfor simultaneously transmitting the data streams onto a network,comprising the steps of: detecting a synchronizing signal from the inputvideo signal; shifting start timings for compression/encoding processescorresponding to the generation of the respective data streams from oneanother in units of frame in accordance with the detected synchronizingsignal; and generating fragmented packets carrying the individual datastreams in accordance with amounts of data generated per unit time bythe respective compression/encoding processes, and transmitting thefragmented packets onto the network at equal intervals within the unittime.